Problem: In math class, the girl to boy ratio is $10$ to $9$. If there are a total of $76$ students, how many girls are there?
Explanation: A ratio of $10$ girls to $9$ boys means that a set of $19$ students will have $10$ girls and $9$ boys. A class of $76$ students has $4$ sets of $19$ students. Because we know that there are $10$ girls in each set of $19$ students, the class must have $4$ groups of $10$ girls each. There is a total of $40$ girls in math class.